A pressure sensor using force sensitive vibrating crystals which can be produced at low cost and used in a wide spectrum of applications has been long sought by industry. Sensors of this type would ideally have an extremely stable scale factor and bias (zero offset), low thermal sensitivity (with appropriate crystal forms), and an output in the form of a variable frequency signal--a format which is well suited for a digital environment.
The output signal from a vibrating crystal transducer is an electrical pulse train which varies in frequency as a load is applied. This form of signal is generally the easiest and most error free to convert to a digital/binary word. A frequency signal is also the easiest signal to integrate (sum over a time period). This can be done by simply counting the number of pulses in the signal over the time period.
A vibrating crystal can give a stable scale factor, which is very important. Unlike many other types of transducers, pressure transducers may experience a full scale input for long periods of time. Operating most of the time at full scale makes any changes in scale factor from the calibration value produce significant errors.
Previously developed and proposed pressure sensors of the vibrating crystal type typically have one or more potential limitations.
One potential limitation is that the bellows and diaphragm characteristics of those sensing devices are not necessarily very repeatable and stable in their deflection response to pressure. They can exhibit creep, hysteresis, and/or sensitivity to the mounting of the completed assembly and to overpressure exposure.
A second limitation common to prior art pressure sensors of the vibrating crystal type is the fairly large no-load frequency change that the single crystal typically employed in those devices undergoes as the temperature of the crystal changes. The larger the temperature sensitivity, the more precise the temperature compensation must be to achieve a given accuracy of pressure measurement.
A third potential problem is that differential thermal expansion of the device's components can increase the sensitivity of the crystal to temperature changes. This is very important as other components of the device and the materials from which they are fabricated need to meet other criteria to produce the required overall characteristics of the pressure sensing device; and an exact thermal expansion match to the crystals could accordingly cause the assembly to be deficient in some other respect.
If only one crystal is used, as is common in prior art pressure sensors of the vibrating crystal type, the accuracy of the signal frequency is very sensitive to the accuracy of the clock in the sensing device or system. This may necessitate the use of an expensive and/or large clock.
A fifth at least potential problem common to prior art devices is high sensitivity to acceleration. Pressure transducers are not necessarily placed in a known position relative to gravity in all applications. They accordingly need to be insensitive to gravity to avoid significant errors in the signals they output.
A sixth potential problem is a result of the attachment of the crystal to a pressure sensing diaphragm at one end and to a fixed member at the other. Creep in the attachment member-crystal joints can affect the high stability and accuracy typically required in those applications for which pressure sensing devices of the vibrating crystal type are supplied.
All of the just-discussed disadvantages of prior art pressure sensors of the vibrating crystal type are addressed by using a sensing crystal and a fixed reference crystal in a push-fixed arrangement and outputting a signal which is the difference between those signals generated by these two crystals. Pressure sensors using vibrating crystals are disclosed in U.S. Pat. No. 4,020,448 issued 26 Apr. 1977 to Corbett; U.S. Pat. No. 4,067,241 issued 10 Jan. 1978 to the same patentee; U.S. Pat. No. 4,382,385 issued 10 May 1983 to Paros; U.S. Pat. No. 4,479,385 issued 30 Oct. 1984 to Koehler; U.S. Pat. No. 4,751,849 issued 21 Jan. 1988 to Paros et al.; and U.S. Pat. No. 5,036,715 issued 6 Aug. 1991 to Hanson. With the exception of the devices disclosed in the Hanson patent, however, those employing a pair of load sensitive crystals have not had the features needed to take full advantage of the stability of vibrating crystals. The Hanson device avoids and accounts for the potential undesirable features of other vibrating crystal type pressure sensing devices but has the disadvantage that it does not readily lend itself to assembly by low cost, mass production techniques.